As shown in FIG. 10, an inverter of a three-phase neutral clamp type has such a structure that an inverter bridge having four switch elements and two clamp diodes per phase is used and an intermediate voltage divided equally by a capacitor having a DC bus voltage connected in series can be output to a phase output terminal. When S1 and S2 are ON and S3 and S4 are OFF in four switch elements connected in series, a phase output terminal voltage is positive. When the S3 and the S4 are ON and the S1 and the S2 are OFF, the phase output terminal voltage is negative. When the S2 and the S3 are ON and the S1 and the S4 are OFF, the phase output terminal voltage is an intermediate voltage. Accordingly, the S1 and S3 and the S2 and S4 are not turned ON at the same time. In many cases, therefore, the ON/OFF signal of the S2 is obtained by inverting the signal of the S1 and that of the S4 is obtained by inverting the signal of the S2.
However, a switching element and a driving circuit have a delay. There is a possibility that respective switches might be simultaneously turned ON by the delay and a large short-circuit current might flow to the switch element to be broken. In consideration of the delay, therefore, a dead time generating circuit shown in FIG. 11 is provided in a controller and the actual ON/OFF signal of the switch element is always provided with a dead time period to be a simultaneous OFF period shown in FIG. 12. APWM generator in FIG. 11 generates PWM pulse signals having respective phases (U1, U2, V1, V2, W1, W2) based on a command of an output voltage created in the controller. In the inverter of the three-phase neutral clamp type, the PWM pulse signal is to be generated by a set of the S1 and S3 and a set of the S2 and S4. For this reason, each of the PWM pulse signals makes a signal such that the S1 and S3 and the S2 and S4 are not turned ON at the same time by an inverting circuit, a delay circuit and an AND gate, and each of the switches is driven in response to an ON/OFF signal so that the switch element can be prevented from being short-circuited. Also in the case in which a load current is equal to or larger than a current which can be caused to flow by the switch element, moreover, there is a possibility that the switch element might be broken. For this reason, it is necessary to provide a protecting device for detecting a current flowing to a switch element and turning OFF the switch to carry out a stop.
Such a protecting method has been proposed in JP-A-10-164854 and JP-A-11-32426.
FIG. 13 is a diagram showing the structure of a power converter disclosed in the JP-A-10-164854. Currents flowing to switching elements 3A to 3D are monitored by short-circuit detecting and breaking circuits 5A and 5B and short-circuit detecting circuits 6A and 6B respectively, thereby detecting a power short-circuit and a load current abnormality. When the abnormality is detected, the switching elements 3A and 3D are turned OFF in a later timing than a normal timing in accordance with the breaking operations of the short-circuit detecting and breaking circuits 5A and 5B and a gate signal sent from a gate control section 16, and then, one of the switching elements 3B and 3C is turned ON and the other is turned OFF in a later timing than the normal timing.
However, a processor is generally used for a controller for controlling an inverter to carry out a control by software. Therefore, a control for suppressing a load current cannot be carried out by the software of the controller on such a condition that the load current rapidly becomes an overcurrent in a short time as in a PWM cycle. For this reason, as for the rapid increase in the load current, the load current is to be suppressed at a high speed or the switching of the inverter is to be stopped without using a processor.
The JP-A-10-164854 and the JP-A-11-32426 have proposed a method of safely stopping switching without breaking the switch element of the inverter in the overcurrent. However, there have been problems in that a rapidly increased load current cannot be controlled and a special delay circuit and a circuit for carrying out a complicated ON/OFF operation are required for the driving signal system of the switch element.